A limited number of golf club heads known as "irons" are still produced by the traditional metal forging process, and some club heads known as "woods" are still produced by machining the heads from solid or laminated blocks of persimmon or other wood. However, the vast majority of golf club heads, both irons and woods, are now made of metal by the investment casting process. This process consists of casting molten metal into a mold around a foam or wax pattern. When the cast metal solidifies, the article is removed from the mold and the foam or wax core is then extracted to form a cavity within it.
In the case of so-called "metal woods," the result of the investment casting process is a hollow, partially complete club head that is completed by welding a bottom sole plate onto the head. It is also common in the art to add foam back into the hollowed portion of the head to provide additional weight, if desired, and to muffle the metallic sound of the metal club upon impact with a golf ball.
The development of metal club heads, particularly metal woods, formed by the investment casting process has greatly improved the overall performance of golf clubs by increasing the distance of drives in fairway shots. Metal woods are harder and stronger, thereby imparting greater power on the golf ball with an equivalent swing. Metal woods have also reduced the problem of distortion and warpage experienced by conventional wooden woods.
However, although the investment casting process has greatly improved the performance of golf clubs, there are inherent difficulties and limitations with golf club heads produced by the investment casting process. For example, pin holes sometimes form in the metal due to air or humidity entrapment in the investment foam. Another problem is that the investment casting process results in the occurrence of non-uniform wall thicknesses due to the uneven flow of the molten metal. Both of the above problems can affect the structural integrity of the golf club head, and frequently result in crinkles and damage when the golf club head is impacted repeatedly upon use.
Another problem inherent in the investment casting process relates to the reproducability or accuracy of products made by the process. As presently practiced, a separate foam or wax pattern for each club made by the investment casting process is made from a master die. This means that there will be slight dimensional variations from pattern to pattern. Such variations are further compounded by the effects of humidity, foam variables, melt chemistry, and pour conditions. These processing variations result in dimensional deviations in the final product which can impact negatively on important engineering characteristics of the club head.
In order to cope with the problems and limitations inherent to the investment casting process, foundries have been forced to resort to a 100% visual inspection of the golf club heads so produced. When defects are found, if fixable at all, secondary rework operations such as welding and grinding are required to correct the problems. This type of piece-by-piece inspection and repair is time consuming and adds to the overall cost of the final product.
The present invention substantially overcomes the problems associated with golf club heads produced by the investment casting process.